The present invention relates to high speed drilling apparatus and, more particularly, to such an apparatus and method for advancing a drill into the workpiece at a rate exactly proportional to the drill rotational speed.
Heretofore the patent literature has included systems such as shown in U.S. Pat. No. 3,446,099 utilizing open loop control for positive feed drilling. Also the prior art literature is illustrative of air turbine drives for cutting tools such as shown in U.S. Pat. No. 2,433,874.
The referenced U.S. Pat. No. 3,446,099 titled "Adaptive Control For A Machine Tool" can be contrasted to the present system which is utilized to drill holes very rapidly and up to ten diameters deep in aluminum while minimizing the energy required and mass of equipment. The present control system positively feeds the drill to optimize chip formation and removal and better control host machine tool loading due to drill thrust loads. Drills are operated at 10 to 20% of breaking torque.
The prior art U.S. Pat. No. 3,446,099 system purports to control the torque on small diameter drills in hard materials below the breaking torque by maintaining a constant feed rate and varying rpm to achieve torque control. Flute packing is sensed through a torque sensor and the drill withdrawn to clear chips thereby permitting drilling to a 150 diameter depth. Spindle inertia in the referenced U.S. Pat. No. 3,446,099 system is held to a minimum to allow timely rpm variation to achieve the purported torque control. In contrast, the effective spindle inertia of the present system is much greater than the minimum and is critical to the operation of the present system. In order to drill aircraft quality fastener holes in aluminum material of the sizes presently desired and at the rate presently desired, 18 plus horsepower would be required. In accordance with the present system such holes require approximately 5 horsepower due to the storage of mechanical energy in the rotating spindle system, primarily in the flywheel like turbine disc. Time between holes found in the drilling operation is utilized in the present system to store up the energy required.
In accordance with a preferred embodiment of the present invention mechanical energy storage provides the following advantages:
1. Smaller and lighter equipment is required than would be necessary with a constant output power source.
2. The present system utilizes less energy than a constant output power source.
Mechanical energy storage as outlined above is especially useful in drilling operations due to the discontinuous nature of drilling as a machining operation. The U.S. Pat. No. 3,446,099 "Adaptive Control For A Machine Tool" utilizes a spindle system wherein spindle inertia is held to a minimum to allow rapid acceleration and deceleration of the drill and makes no attempt at mechanical energy storage--precisely the opposite of what is desired in accordance with the teachings of the present system.
The input horsepower required of the present system is a function of the total time allowed for the drilling cycle, including time between holes, rather than that required of the drill purely during the time of cutting as in the prior art U.S. Pat. No. 3,446,099 system.
The aforementioned U.S. Pat. No. 3,446,099 system is continuously coupled, the spindle to the feed, or completely cut off through a switch. This means the stepper motor or motors used in the feed system drive is or are required to start and stop within one step to remain in the positive feed desired. Therefore the stepper motor is running in its base speed range. Accordingly, increased speed is attained by coupling two stepper motors to a different gear train and alternatively feeding each thereby effectively doubling the speed capability.
In contrast the present system utilizes a spindle which is kept constantly turning at a high energy rotational rate. The feed system remains stationary until actuation, whereon the ramp generator delivers, at rapidly and linearly increasing frequency, a pulse train to the stepper, accelerating it to approximately ten times the base speed. When the frequency of the pulse train from the ramp generator is equal to the frequency of the pulse train from the spindle source, i.e. synchronism, the present circuit switches from delivering pulses from the ramp generator to the stepper motor to delivering pulses from the spindle source to the stepper motor, placing the stepper motor under direct spindle control and thereby achieving positive feed.
The present system could not function without the aforementioned ramping and switching since the required motor speed is in the slewing speed range of stepper motor operation. Accordingly the present system could not function through simply an on/off switching arrangement as done in the prior art U.S. Pat. No. 3,446,099 system.
During the time the stepper motor is under control by the spindle pulse train, the ramp oscillator frequency is caused to oscillate up and down rapidly and thus held very close to the spindle frequency by causing the ramp frequency to increase when below the spindle frequency and decrease when above the spindle frequency. Were the ramp frequency not so controlled, it would always go to some maximum frequency which could be much higher than the spindle frequency. In such a case there would be a finite time delay from the time the ramp down or drill retract signal was received and the time when the ramp generator decreased in frequency sufficiently to reassume control of the stepper motor from the spindle pulse train, i.e. until synchronism. This would cause unacceptable drill feed overtravel.
As the U.S. Pat. No. 3,446,099 system operates in the much lower base speed range of stepper motor operation, no such ramp generator or ramp generator control is required. As the present system operates at approximately ten times base speed, were a simple switching on/off system used as in the U.S. Pat. No. 3,446,099 system, approximately ten stepper motors coupled through differential gear trains would be required to achieve the feed rate currently desired.
The prior art U.S. Pat. No. 3,446,099 system incoporates a torque sensor on the spindle whereas none is used in the present system. In the U.S. Pat. No. 3,446,099 system this feature is used to control drill torque whereas the present system does not attempt to control by torque required of the drill.
In accordance with a preferred embodiment of the present invention a conventional drill is rotated by a minimally limited inertia air turbine motor. The drill and the turbine motor are moved back and forth (moving the drill into and out of the workpiece) on a precision linear travel feed carriage. The carriage is moved by a preloaded ball screw coupled to a stepper motor by a toothed belt. A unique feature of the present system is that the drill advances into the workpiece at a rate exactly proportional to the drill rotational speed. In accordance with the present system, the coupling between the drill rotary and linear movement is done through the present open loop control system. In testing the current system demonstrated drilling of aircraft quality fastener holes at rates better than twenty times faster than any commercially available positive feed drilling system.